1. Field of the Invention
The present invention relates to a semiconductor fabricating apparatus. More particularly, the present invention relates to a cassette table for supporting a wafer cassette outside a processing chamber.
2. Description of the Related Art
In a semiconductor fabricating system, a spinner or a scrubber rapidly rotates a wafer that is being processed. Static electricity occurs on the fast-rotating wafer due to friction between the wafer and air or de-ionized water (DI water). When the wafer is charged with static electricity, large numbers of particles will electro-statically adhere to the wafer. Also, arcing occurs during wafer handling. The arcing can damage wiring patterns or metal lines of the wafer. In particular, the larger the wafer is, the greater the static electricity affects the wafer in these ways.
These problems are exacerbated by a conventional cassette table having non-conductive cassette supporters, as shown in FIG. 1. After a spinner or a scrubber rotates a wafer in carrying out a coating or a cleaning process, the wafer charged with static electricity by the process is transferred to a wafer cassette. Wafer support pins and wafer-contacting parts of the wafer transfer device (not shown) are made of urethane or another soft non-conducting material in order to avoid scratching the wafer. Therefore, the static electricity accumulated on the wafer is not discharged through the wafer transfer device and the support pins. The cassette 20 into which the charged wafer is loaded is disposed on a cassette table 10. As wafers are sequentially loaded into the cassette 20, the amount of static electricity applied to the cassette 20 increases.
The cassette 20 is made of a static dissipative material. Thus, the cassette 20 could discharge static electricity to a grounded conductor. However, although the cassette table 10 includes a top plate 12 of a conductive material, the cassette table 10 also includes a cassette supporter 14 of a non-conductive material. The cassette supporter 14 comprises one or more guide blocks 18 for positioning the cassette 20 on the table 10, and one or more pads 16 on which the cassette 20 sits. The static electricity accumulated on the cassette 20 is not discharged through the grounded top plate 12 because the cassette supporter 14 is entirely non-conductive.
The guide blocks 18 and the pads 16 are necessary components of the cassette table 10 in a semiconductor fabricating system. The guide blocks 18 position the cassette 20 relative to a downstream wafer transfer device and suppress the mobility of the cassette 20. The pads 16 alleviate the shock that the cassette 20 would otherwise experience while disposed on the top plate 12 of the cassette table 10.
The guide blocks 18 and the pads 16 must be made of soft materials such as non-conductive Teflon to alleviate shock and prevent the cassette 20 from being scratched and abraded. If the cassette supporter 14 were made of a conductive material, the static electricity accumulated on the cassette 20 would readily discharge to the top plate 12. However, friction between the cassette 20 and the hard material of the cassette supporter 14 would scratch or abrade the cassette 20. Such abrasions and scratches could cause the cassette 20 to deviate from its predetermined position on the cassette supporter 14. Such deviation could result in a collision between a blade and of the wafer transfer device and a wafer when the wafer transfer device picks the wafer from the cassette 20.
FIG. 2 illustrates wafers to which particles adhere during processing. More particularly, FIG. 2(a) shows a wafer form which static electricity has been discharged, whereas FIG. 2(b) shows a wafer charged with static electricity. As the figures show, many more particles adhere to a wafer charged with static electricity than to a wafer from which static electricity has been discharged. This unfavorable phenomenon is more pronounced in wafers having large diameters.
FIG. 3 illustrates a portion of a wafer on which metal lines are formed. When a wafer charged with the static electricity is handled, arcs are created and the arcs damage portions (a) of the metal lines of the wafer. A wafer having damaged portions (a) of its metal lines will be rejected during an inspection process. Thus, arcing as the result of static electricity accumulated on a wafer lowers the manufacturing yield.
Still further, the static electricity accumulated on the cassette generates electrical interference in the semiconductor fabricating system. The electric interference causes the semiconductor fabricating system to malfunction.
One object of the present invention is to provide a cassette table of a semiconductor fabricating apparatus that allows static electricity to discharge from a cassette.
Another object of the present invention is to provide a cassette table that allows static electricity to discharge from a cassette without scratching or abrading the cassette.
A cassette table according to the present invention includes a top table, and a cassette supporter that is mounted on the top plate and supports a cassette above the top table, the cassette supporter including discharge means for discharging static electricity from the cassette.
The cassette supporter has one or more guide blocks for guiding the cassette into a predetermined position on the top plate, and one or more conductive pads on which the cassette rests.
In one embodiment, the guide blocks for guiding the cassette to a predetermined position on the top plate and the pads on which the cassette is located are electrically non-conductive members. The discharge means is disposed on at least one of the guide block(s) and the pad(s).
The top plate is grounded, and the discharge means electrically connects the cassette to the top plate. Preferably, the discharge means is a conductive cover for covering at least one of the non-conductive guide block(s) and the non-conductive pad(s). The cover may be made of a material consisting of 70 wt. % polyether ether ketone (PEEK) and 30 wt. % carbon fibers.
In another embodiment, the cassette table includes a top plate and a cassette supporter consisting of a soft electrically conductive material. In this case, therefore, the guide blocks and the pads of the cassette supporter are of the soft electrically conductive material. Preferably, the material is a combination of 70 wt. % polyether ether ketone (PEEK) and 30 wt. % carbon fibers. Again, the top plate is grounded, and the cassette supporter is mounted upon the top plate so as to allow static electricity to be discharged from the cassette.